This invention relates to the field of multichannel audio. More particularly the invention relates to matrix-encoded surround-sound channels in a discrete typically digital sound format for motion picture soundtracks.
Optical soundtracks for motion pictures were first demonstrated around the turn of the century, and since the 1930""s have been the most common method of presenting sound with motion pictures. In modern systems, the transmission of light through the film is modulated by variations in soundtrack width, where an ideally transparent varying width of soundtrack is situated within an ideally opaque surrounding. This type of soundtrack is known as xe2x80x9cvariable areaxe2x80x9d.
In an effort to reduce distortion due to non-uniform light over the soundtrack width and other geometric distortion components, the xe2x80x9cbilateralxe2x80x9d variable area track was introduced. This format has two modulated edges, identical mirror images around a fixed centerline. A later development, which is now the standard monophonic analog optical soundtrack format, is called xe2x80x9cdual bilateralxe2x80x9d (or xe2x80x9cdouble bilateralxe2x80x9d or xe2x80x9cduo-bilateralxe2x80x9d) sound track. This format has two bilateral elements within the same soundtrack area, thus providing further immunity from illumination non-uniformity errors. A useful discussion of the history and potential of optical soundtracks can be found in xe2x80x9cThe Production of Wide-Range, Low-Distortion Optical Sound Tracks Utilizing the Dolby Noise Reduction Systemxe2x80x9d by Ioan Allen in J. SMPTE, September 1975, Volume 84, pages 720-729.
In the mid 1970""s Stereo Variable Area (SVA) tracks became increasingly popular, in which two independently modulated bilateral soundtracks are situated side by side in the same area as the normal monophonic (mono) variable area track.
In 1976, Dolby Laboratories introduced its four-channel stereo-optical version of Dolby Stereo, which employed audio matrix encoding and decoding in order to carry 4 channels of sound on the two SVA optical tracks. xe2x80x9cDolbyxe2x80x9d and xe2x80x9cDolby Stereoxe2x80x9d are trademarks of Dolby Laboratories Licensing Corporation. Dolby Stereo for SVA optical tracks employs the xe2x80x9cMPxe2x80x9d matrix, a type of 4:2:4 matrix system that records four source channels of sound (left, right, center and surround) on the two SVA tracks and reproduces four channels. Although the original Dolby Stereo stereo-optical format employed Dolby A-type analog audio noise reduction, in the mid-1980""s Dolby Laboratories introduced an improved analog audio processing system, Dolby SR, which is now used in Dolby Stereo optical soundtrack films.
Multichannel motion picture sound was employed commercially at least as early as xe2x80x9cFantasoundxe2x80x9d in which the four-channel soundtrack for the motion picture Fantasia was carried in respective optical tracks on a separate film synchronized with the picture-carrying film. Subsequently, in the 1950s, various xe2x80x9cmagnetic stripexe2x80x9d techniques were introduced in which multiple channels of sound were recorded in separate tracks on magnetizable materials affixed to the picture-carrying film. Typically, magnetic striped 35 mm film carried three or four separate soundtracks while magnetic striped 70 mm film carried six separate soundtracks.
Although most motion picture films with magnetic striped soundtracks carried a separate channel in each magnetic track, at least one film released in the mid-1970s (Tommy in xe2x80x9cQuintaphonicxe2x80x9d sound) employed matrix encodingxe2x80x94the normally left and right tracks were matrix encoded with left front, left rear, right front and right rear sound channels. The third, center channel remained discrete. The phase sensitive matrix system suffered from sound image wandering due to variations in phasing between the matrix-encoded tracks.
In a variation of PerspectaSound used in some prints of the motion picture Around the World in Eighty Days, four magnetic tracks on 35 mm carried left, center, right and surround channel information, respectively. In addition to the surround information, the fourth track carried subaudible tones for directing the surround sound to a selected bank of three banks of surround sound loudspeakers. Early forms of PerspectaSound employed a subaudible control tone on the monaural soundtrack in order to direct the sound to selected loudspeakers behind the screen.
Magnetic striped 35 mm films became obsolete after the introduction of the Dolby Stereo 35 mm optical format.
In another version of Dolby Stereo introduced in the 1970s, Dolby noise reduction was applied to four of the six discrete audio tracks of magnetic striped 70 mm motion picture film. As a feature of this Dolby Stereo format, tracks 1 and 2 (recorded in the magnetic stripe located between the left edge of the film and the left-hand sprocket holes) carry the left main screen channel and low-frequency-only xe2x80x9cbass extensionxe2x80x9d information, respectively; track 3 (recorded in the magnetic stripe located between the left-hand sprocket holes and the picture) carries the center main screen channel; track 4 (recorded in the magnetic stripe located between the picture and the right-hand sprocket holes) also carries low-frequency-only xe2x80x9cbass extensionxe2x80x9d information; and tracks 5 and 6 (recorded in the magnetic stripe located between the right sprocket holes and the right edge of the film) carry the right main screen channel and the single surround channel, respectively. Dolby noise reduction is not applied to the bass extension information.
In a variation of Dolby Stereo for 70 mm magnetic soundtrack motion picture films, two surround channels are provided instead of one (referred to as xe2x80x9csplit surroundsxe2x80x9d or xe2x80x9cstereo surroundsxe2x80x9d). Tracks 1, 3, 5 and 6 are the same as in conventional Dolby Stereo 70 mm; however, mid- and high-frequency left surround information is recorded (with Dolby noise reduction) in track 2 along with the low-frequency bass information, and mid- and high-frequency right surround information is recorded (with Dolby noise reduction) in track 4 along with the low-frequency bass information. When reproduced in a theater, the mid- and high-frequency stereo surround information on tracks 2 and 4 is fed to the left and right surround speakers, respectively, combined with monophonic surround bass information from track 6. This variation of Dolby Stereo 70 mm was an early form of the now-common xe2x80x9c5.1xe2x80x9d channel (sometimes referred to as six channel) configuration: left, center, and right main screen channels, left and right surround sound channels and a low-frequency bass enhancement (LFE) or subwoofer channel. The LFE channel, which carries much less information than the other full-bandwidth channels, is now referred to as xe2x80x9c0.1xe2x80x9d channels.
In spite of these advances in analog soundtrack fidelity, film soundtracks had long been considered a candidate for digital coding due to the high cost of 70 mm magnetic soundtrack films and the perceived limitations of the matrix technology employed in 35 mm optical soundtrack films. In 1992, Dolby Laboratories introduced its Dolby Digital optical soundtrack format for 35 mm motion picture film. Dolby Digital is a trademark of Dolby Laboratories Licensing Corporation. 5.1 channel (left, center, right, left surround, right surround and LFE) soundtrack information is digitally encoded employing Dolby Laboratories AC-3 perceptual encoding scheme. That encoded information is in turn encoded as blocks of symbols optically printed between the film""s sprocket holes along one side of the film. The analog SVA tracks are retained for compatibility. Details of the Dolby Digital 35 mm film format are set forth in U.S. Pat. Nos. 5,544,140, 5,710,752 and 5,757,465. The basic elements of the Dolby AC-3 perceptual coding scheme are set forth in U.S. Pat. No. 5,583,962. Details of a practical implementation of Dolby AC-3 are set forth in Document A/52 of the United States Television Systems Committee (ATSC), xe2x80x9cDigital Audio Compression Standard (AC-3),xe2x80x9d Dec. 20, 1995 available on the world wide web of the Internet. The Dolby Digital system typically provides the channel discreteness of 70 mm magnetic soundtrack films while preserving the low cost and compatibility of 35 mm optical soundtrack films.
Subsequently, in 1993, Sony introduced its Sony Dynamic Digital Sound (SDDS) format for 35 mm motion picture film. In the SDDS system xe2x80x9c7.1xe2x80x9d channel (sometimes referred to as eight channel) (left, left center, center, right center, right, left surround, right surround and LFE) soundtrack information is digitally encoded using a form of Sony""s ATRAC perceptual coding. That encoded information is in turn encoded as strips of symbols optically printed between each edge of the film and the nearest sprocket holes. Sony, Sony Dynamic Digital Sound, SDDS, and ATRAC are trademarks. Some details of the Sony SDDS system are set forth in U.S. Pat. Nos. 5,550,603; 5,600,617; and 5,639,585.
Also in 1993, Digital Theater Systems Corporation (xe2x80x9cDTSxe2x80x9d) introduced a separate medium digital soundtrack system in which the 35 mm motion picture film carries a time code track for the purpose of synchronizing the picture with a CD-ROM encoded using a type of perceptual coding with 5.1 channel soundtrack information (left, center, right, left surround, right surround and LFE). DTS is a trademark. Some details of the DTS system are set forth in U.S. Pat. Nos. 5,155,510; 5,386,255; 5,450,146; and 5,451,942.
Further details of the Dolby Digital, Sony SDDS and DTS systems are set forth in xe2x80x9cDigital Sound in the Cinemaxe2x80x9d by Larry Blake, Mix, October 1995, pp. 116, 117, 119, 121, and 122.
FIG. 1 shows an idealized loudspeaker arrangement for a typical theater 10 employing the Dolby Digital or the DTS 5.1 channel systems. The left channel soundtrack L is applied to left loudspeaker(s) 12, the center channel soundtrack C is applied to the center loudspeaker(s) 14 and the right channel soundtrack R is applied to the right loudspeaker(s) 16, all of which loudspeakers are located behind the motion picture screen 18. These may be referred to as main screen channels. The left surround channel LS is applied to left surround loudspeaker(s) 20 shown at the rear portion of the left wall 22 of the theater. The right surround channel RS is applied to right surround loudspeaker(s) 24 shown at the rear portion of the right wall 26 of the theater. In normal practice, there are a plurality of left surround loudspeakers spaced along the left side wall of the theater starting from a location about midway between the front and rear of the theater and extending to the rear wall 28 and then along the rear wall to a location near the mid-point of the rear wall. The right surround loudspeakers are arranged along the along the right side wall and rear wall in a mirror image of the left surround loudspeaker arrangement. In addition, low frequency effect (LFE) or subwoofer loudspeakers (not shown), carrying non-directional low frequency sound, are usually located behind the screen 18, but may be located elsewhere. For simplicity, no LFE or subwoofer loudspeakers are shown in any of the drawings.
FIG. 2 shows an idealized loudspeaker arrangement for a typical theater 10 employing the Sony SDDS 7.1 channel system. The arrangement is the same as shown in FIG. 1 for the Dolby and DTS systems with the exception that the Sony SDDS system provides two additional main screen channelsxe2x80x94a left center channel LC that is applied to left center loudspeaker(s) 13 and a right center channel RC that is applied to right center loudspeaker(s) 15.
All three digital motion picture sound systems provide at least three discrete main screen channels and two discrete surround sound channels. Although two surround sound channels are sufficient to satisfy the creators of and audiences for most multichannel sound motion pictures, there are, nevertheless, desires for more than two surround sound channels for some motion pictures.
The desire for more than two surround sound channels is addressed in two related patents (U.S. Pat. Nos. 5,602,923 and 5,717,765) that disclose an approach for providing additional surround-sound channels to the 7.1 channel Sony SDDS system. The patents point out that the SDDS system is xe2x80x9cpushing the bandwidth limits of current motion picture technology in order to obtain the eight channels of informationxe2x80x9d and that xe2x80x9cadditional tracks are beyond the current practical bandwidth available on conventional motion picture film unless main or surround channel bandwidth is sacrificed.xe2x80x9d
The U.S. Pat. Nos. 5,602,923 and 5,717,765 patents add one or more very high frequency tones to the left surround and right surround channels in order to direct all or a portion of the information in a respective surround channel from the normal left surround and right surround loudspeakers to loudspeakers above the audience and above the motion picture screen. However, a shortcoming of that approach is its inability to reproduce different surround sound channels simultaneously from each of the more than two banks of surround sound loudspeakers. In other words, at any one time there are only two possible surround sound channels even though the loudspeaker locations that produce those channels may be varied.
Accordingly, there is still an unfulfilled need to provide more than two surround sound channels within the current formats of the Dolby Digital, Sony SDDS and DTS digital soundtrack systems.
It is an object of the present invention to provide more than two surround sound channels within the format of a digital soundtrack system designed to provide only two surround sound channels.
It is an object of the present invention to provide more than two surround sound channels within the format of a digital motion picture soundtrack system designed to provide only two surround sound channels, whether recorded on film or another medium synchronized with the picture.
It is another object of the present invention to provide more than two surround sound channels within the same digital audio stream of a digital soundtrack system designed to provide only two surround sound channels.
It is another object of the present invention to provide more than two surround sound channels in a digital soundtrack system designed to provide only two surround sound channels such that the ability of existing playback equipment to play two surround sound channels is unaffected.
It is another object of the present invention to provide more than two surround sound channels within the format of a digital soundtrack system designed to provide only two surround sound channels such that the medium carrying the digital soundtracks appears unaltered.
It is yet another object of the present invention to employ matrix encoding and decoding only for surround sound motion picture channels, which channels are relatively immune to crosstalk in matrix encoding and decoding.
It is yet a further object of the invention to employ discrete soundtrack channels for main screen sound channels where crosstalk is undesirable, but to employ matrix encoding for surround sound soundtrack channels where crosstalk is more acceptable due to the rearward directional characteristics of the human ear.
It is still a further object of the invention to employ discrete soundtrack channels for main screen sound channels where crosstalk is undesirable, but to employ matrix encoding for surround sound soundtrack channels where crosstalk is more acceptable due to the location of the surround sound channel loudspeakers in adjacent quadrants where crosstalk is relatively benign.
It is yet a further object of the invention to provide more than two surround sound channels in a digital motion picture soundtrack system which carries the digital information as optical symbols printed on motion picture film without increasing the density of optical symbols above that required to carry only two surround sound channels.
In accordance with the present invention more than two surround sound channels are provided within the format of a digital soundtrack system designed to provide only two surround sound channels. The digital audio stream of the digital soundtrack system designed to provide only two surround sound channels remains unaltered, thus providing compatibility with existing playback equipment. Moreover, the format of the media carrying the digital soundtracks is unaltered. In the case of the Dolby Digital and Sony SDDS systems, the digital information carrying symbols printed on the motion picture film remain unchanged thexe2x80x94symbol or xe2x80x9cbitxe2x80x9d size need not be reduced. In both the Dolby Digital and Sony SDDS systems, the optically recorded symbols represent digital information and the digital information, in turn, represents discrete motion picture soundtrack channels. Although, in theory, additional channels could be carried by reducing the symbol size in order to provide more bits and allowing the storage of more data in the same physical area, such a reduction would introduce unwanted difficulties in the printing process and require substantial modification of recorder and player units in the field. (In the same way, Dolby Laboratories rejected the approach of putting data on the opposite side of the film, with all the technical and economic problems that would ensue.) Full backward and forward compatibility is maintained.
In addition, the xe2x80x9cdiscretenessxe2x80x9d of the digital soundtrack system is not audibly diminished by employing matrix technology to surround sound channels, particularly if active matrix decoding is employed. The human ear""s relative insensitivity with respect to rearward-originating sounds compared to the ear""s sensitivity to forward-originating sounds makes the use of matrix encoding for surround channels highly acceptable in an otherwise discrete channel reproduction system (any crosstalk among surround channels behind the head is likely not to be perceived by the listener; moreover, crosstalk among channels in the same adjacent quadrants is more acceptable than crosstalk, for example, from the dialog-carrying center channel to the surround channel in an LCRS matrix system).
These and other objects, advantages and features of the invention will become apparent to those skilled in the art upon consideration of the present specification, drawings and claims.
Aspects of the invention include (1) a digital or analog format medium, such as motion picture film, magnetic tape, optical disc, or magneto-optical disc carrying discrete motion picture soundtracks in which two discrete surround-sound channels are matrix encoded with three, four or five surround-sound channels; (2) a method of producing motion picture soundtracks in which at least three main channels are recorded in discrete soundtrack channels and in which three, four or five surround-sound channels are matrix-encoded and recorded in two discrete surround-sound soundtrack channels; (3) a method of reproducing a motion picture soundtrack carried in at least five discrete motion picture soundtrack channels in which the discrete channels include two surround-sound channels, the two discrete surround-sound channels carrying three, four or five surround-sound matrix-encoded channels; and (4) apparatus for reproducing a motion picture soundtrack carried in at least five discrete motion picture soundtrack channels in which the discrete channels include two surround-sound channels, the two discrete surround-sound channels carrying three, four or five surround-sound matrix-encoded channels.